1. Field of the Invention
This invention relates to electrochemical devices, more particularly to electrochemical devices in which a carbonaceous material undergoes oxidation to produce chemicals and/or electricity. This invention also relates to a method for substantially preventing the crossover of some of the carbonaceous material from one side of the electrolyte through the electrolyte to the other side of the electrolyte of the electrochemical devices. More particularly, this invention relates to direct methanol fuel cells and direct methanol fuel cell anode electrode catalysts, which are both proton and electron conductive and which reduce methanol crossover, a common problem in direct methanol fuel cells.
2. Description of Related Art
An electrochemical device is a device in which a chemical or chemical compound is modified by electronic means to produce other chemicals and/or electricity. Exemplary of devices which produce electricity are fuel cells, which comprise an anode electrode, a cathode electrode and an electrolyte disposed between the anode electrode and the cathode electrode, in which a fuel such as hydrogen or carbonaceous materials such as methane, methanol, ethane, butane, etc. is introduced into the anode side of the electrolyte and an oxidant, such as air, is introduced into the cathode side of the electrolyte and the fuel and oxidant are reacted, resulting in the generation of electricity. Typically, the carbonaceous fuels are first reformed to produce hydrogen, which is then introduced into the fuel cell. However, it will be apparent that fuel cells which are capable of direct utilization of carbonaceous fuels are a desirable objective since the need for reforming would be eliminated.
There exist different types of fuel cells defined, in part, on the basis of the type of electrolyte employed. Molten carbonate fuel cells employ molten carbonates disposed in an electrolyte matrix as an electrolyte; phosphoric acid fuel cells employ phosphoric acid as an electrolyte; solid oxide fuel cells employ solid oxide electrolytes; and polymer electrolyte membrane fuel cells (also referred to as proton exchange membrane fuel cells) employ, as the name suggests, polymeric membranes as an electrolyte.
Direct methanol polymer electrolyte membrane fuel cells are prime candidates for both vehicular and stationary uses due to their inherent simplicity (no external reformers) and potential high energy densities (liquid fuels). In addition, direct methanol polymer electrolyte membrane fuel cells have the potential for replacing rechargeable batteries due to the possibility of a zero recharge time. However, the current state of the art in direct methanol polymer electrolyte membrane fuel cells requires external means, such as pumps and blowers for introducing reactants into and removing reaction products from the fuel cell. For example, U.S. Pat. No. 5,573,866 to Van Dine et al. teaches a polymer electrolyte membrane fuel cell which directly oxidizes liquid methanol fuel that is fed into the anode chamber from a liquid methanol storage container. The liquid methanol is mixed with water in the anode chamber. Some of the methanol and water cross over the membrane into the cathode chamber and into a process air stream. The methanol and water are removed from the cathode chamber by evaporation into the process air stream, which is then directed into a condenser/radiator. The methanol and water vapors are condensed in the condenser/radiator, from whence the condensed water and methanol are returned to the anode chamber of the cell. The evaporating cathode process air stream, which is provided to the cathode chamber by means of a fan, provides oxygen for the fuel cell reaction, and also cools the cell.
Direct methanol fuel cells (DMFCs) are currently being investigated for a number of different applications from milliwatt to kilowatt scale. The most common obstacles are the lack of catalyst activity at the anode and the inability of the membrane electrolyte to be an effective methanol barrier. Numerous concepts have been promoted for reducing methanol crossover from the anode to the cathode. These include (1) increasing membrane thickness, which disadvantageously increases the internal resistance of the cell, (2) modifying the existing membrane with organic or inorganic materials to form a physical obstacle to hinder methanol crossover, which disadvantageously jeopardizes the performance or stability of the membrane, and (3) finding new polymers that provide high proton conductivity and low methanol crossover, which to date has not been achieved.